1. Technical Field
The present invention relates to rotors used within gas turbine engines in general, and to gas turbine rotors having a hollow disk in particular.
2. Background Information
Alternating stator and rotor stages form the fan, compressor, and turbine in most modern gas turbine engines. The stator stages increase the efficiency of the engine by guiding core gas flow into or out of the rotor stages. The rotor stages in the fan and compressor add work to the core gas flow to produce thrust. The rotor stages in the turbine, in contrast, extract a portion of that work to power the fan and compressor. Each rotor stage includes a disk and a plurality of rotor blades mechanically attached (e.g., by "fir tree" or "dovetail" attachment) or integrally bonded (i.e., an "integrally bladed rotor", or "IBR") to the rim of the disk. The rim of the disk is attached to a center hub via a web extending therebetween.
During operation, each rotor disk is mechanically and thermally loaded. Thermal loading (and consequent stress) is usually greatest during a transient period when the disk is exposed to a significant thermal change in a short period of time. Conventional solid rotor disks having a massive hub are particularly susceptible to thermally induced stress. The exterior regions of the massive hub will cool off or heat up relatively quickly in response to the thermal change. The interior regions cannot react as quickly, however, thereby causing a disparity in thermal growth within the hub that results in thermally induced stress. Mechanical loading, the other principal stress inducing component, emanates from core gas acting on the rotor blades and centrifugal force acting on each rotor stage component. The centrifugal force may be simplistically described by the following equation: ##EQU1## where: F=centrifugal force acting on a body; m=the mass of the body; v=the tangential velocity of the body at a particular radial distance; r=the radial distance between the body and the axis of rotation; and .omega.=the angular speed of the body. As can be seen from the above equation, centrifugal force is directly related to the radial distance of the body and the square of the angular speed of the body. Mass of each rotor stage component and its radial position is, therefore, of paramount importance in high speed applications (i.e., those above 20,000 rpm).
Under ideal conditions, the mechanical load is uniformly distributed over the length of the hub bore so that load induced circumferential stress (also called "hoop stress") is likewise uniformly distributed and consequently minimized. Conventional solid rotor disks, however, tend to localize the mechanical load within the center region of the hub (i.e., in the region substantially radially aligned with the web and rotor blades). The hub bore of the conventional solid disk, which ultimately bears the entire mechanical load, consequently experiences a concentration of hoop stress within the center region. A person of skill in the art will recognize that non-uniform load distribution and consequent hoop stress undesirably limits component life.
Higher rotor disk loads have historically been accommodated by increasing the cross-sectional area of the disk; i.e., making the disk more robust. In theory, a greater cross-sectional area promotes distribution of the load within the disk, which in turn minimizes the stress at any particular point. As one might expect from the above explanation of centrifugal force, however, this approach has limits. Increasing the disk cross-sectional area predominantly in the radial direction (see FIG. 3) provides limited returns because of the additional centrifugal load it creates. Increasing the disk cross-sectional area predominantly in the axial direction (see FIG. 4), on the other hand, also provides limited returns because the above described non-uniform loading within the hub.
Adding mass to the disk can also make it difficult to manufacture a disk with uniform mechanical properties. Rotor disks, particularly those used in high speed applications are often forged because forging provides a higher tolerance to hoop stress than a similar cast rotor. After machining, the forged disk is typically heated and rapidly quenched to increase the disk's hoop stress capacity. In some instances, however, the thermal inertia inherent in a massive disk hub prevents the hub's interior region from being quenched at the same rate as its exterior regions. The disparity in quench rates creates a grain structure profile (and consequent material strength profile) exactly inverse to what is needed within a conventional solid disk where the localized load extends through the center region of the hub.
The need to provide cooling air to the rotor blades is another design consideration. Turbine rotor blades, for example, include internal cooling air passages for heat transfer purposes. Providing the cooling air to the rotating blades has historically been a challenge. U.S. Pat. No. 3,742,706 ('706), issued to the General Electric Company, discloses that a rotor disk may comprise two axially spaced disks interconnected by a plurality of circumferentially spaced vanes, which extend radially outward. The vanes and the passages therebetween are claimed to centrifuge and pump cooling air radially outward toward the rotor blades. U.S. Pat. No. 3,982,852 ('852), also issued to the General Electric Company, discloses that interconnecting vanes, similar to those disclosed in '706, can be stressed compressively far in excess of the material capability. A problem with creating passages between disk halves via a plurality of vanes is, therefore, the effect the passages have on the load capacity (and therefore the rotational speed) of the disk. As stated earlier, the loading on conventional disks is axially centered within the disk. Passages aligned with the load path may compromise the load capacity of such a disk.
Another problem with a rotor disk having a plurality of passages between disk halves is the difficulty and cost of manufacturing such a disk. A forged bond between metallic elements can be accomplished in a relatively short period of time, provided the elements to be joined are adequately heated and a high pressure is available to force the elements together. An advantage of a forged bond is that the time at temperature is usually not great enough to cause significant degradation of mechanical properties. A disadvantage of a forged bond, particularly when one of the bond surfaces is a narrow vane, is that one or more vanes could buckle during the bonding process. If none of the vanes buckle, there is still a likelihood that a significant amount of material upset will be produced adjacent the bond interface. A person of skill in the art will recognize that removing upset from radial passages within a rotor disk is difficult at best. Upset left in the passage will impede flow and create undesirable stress risers. A diffusion bond between metallic elements, in contrast, requires a relatively long period of time and adequate heat to create an acceptable bond, but does not require high pressure to force the elements together. An advantage of diffusion bonding is that little or no upset is formed adjacent the bond interface. A disadvantage of using diffusion bonding to join the rotor disk halves is the required time at temperature is usually sufficient to cause significant degradation of mechanical properties.
Hence, what is needed is a rotor disk for a gas turbine engine having a high load capacity, one capable of performing in a high speed application, one that can be successfully heat treated, one that can be readily manufactured, and one capable of providing cooling air to rotor blades attached thereto.